The tunable pore structure in porous carbon materials makes them ideal candidates for a variety of applications, including, for example, adsorption and separation, catalysis, energy storage/conversion, and electrochemical sensors. In applications where it is desirable to have a rapid diffusion of molecules into a porous carbon internal surface—for example, in adsorption and catalytic reactions—it is desirable to employ materials containing mesopores, i.e., materials containing pores with diameters between 2 and 50 nm.
Conventionally, mesoporous carbon (MC) is synthesized by a nanocasting method, often called a hard-template method, where a carbon precursor is introduced into the pores of an ordered mesoporous silica by impregnation, followed by in-situ carbonization and removal of the silica template. This hard-templating method is time-consuming, costly, and unsuitable for scale-up production. In particular, the necessity to remove the inorganic template by employing hazardous chemicals such as HF and NaOH is an unavoidable drawback. Alternatively, direct soft-templating of the carbon material has been developed to prepare MC, which avoids the use of hazardous chemicals employed for the template removal. The soft-template method involves the cooperative assembly of structure-directing agents that are able to form lyotropic phases with suitable organic carbon precursors. Since the first successful synthesis of ordered MC by soft-templating with amphiphilic block-copolymers in 2004, extensive research has been conducted to fabricate different pore structures including but not limited to p6mm, Im3m, Ia3d, Fm3m, and Fd3m. However, the major concern in employing soft-templating methods comes from the utilization and/or release of hazardous reactants such as formaldehyde and phenol during MC preparation. In fact, formaldehyde is often used as a cross-linking agent in MC synthesis, which is known to be carcinogenic and should be removed from industrial processes. Moreover, phenol is also carcinogenic and its derivatives are mostly toxic. Thus, it remains a great challenge so far to prepare MC materials by using “friendly” reagents or from a completely green process.
Utilizing biomass (raw resources such as wood, cotton or treated resources including cellulose, lignin, tannin and starch) as the carbon precursor to synthesize MC seems a promising approach. For example, tannin has been utilized as a precursor to synthesize ordered mesoporous carbon through a soft-template method. Nanocrystalline cellulose has been used as precursor to fabricate mesoporous carbon as well.
Even though both tannin and cellulose are widely accessible, it is not necessarily a green process to produce them. For example, NaOH (corrosive alkali) treatment is often used to extract the cellulose by removing lignin and hemicellulose from raw materials. Currently, it is the understanding of the current inventors of the present invention that all current methods rely on the use of certain chemicals to synthesize MC. From the current state of the art it appears it would be unrealistic to synthesize MC from a bottom-up approach without using chemicals.
FIG. 1 provides a general schematic of the basic structure of natural wood 10. The basic skeletal structure of natural wood 10 consists of arrays of tubular columns known as tracheid 12, which are formed primarily from the cell walls 13 of the arrayed plant cells. As can be seen in FIG. 1, these tracheids 12 have a central opening referred to as a lumen 14 and are held together by a lignin lamella 16. In the mature cell, the cell walls 13 are aggregated into strand-like units of structure called cellulose aggregates or microfibrils 18, formed primarily of a cellulose framework and hemicellulose matrix and having an average diameter of about 16 nm. The areas between these cellulose aggregates 18 define elliptical spaces 20 that have a length/width ratio of ˜2 and a minor diameter across the ellipse of 5-10 nm. Once these elliptical spaces 20 in the cellulose/hemicellulose frame 22 are formed, they are then filled by lignin molecules 24 to form the final structure.
It is technically difficult to remove lignin without damaging cellulose framework from conventional chemical dissolution method. The currently practiced acid or alkali hydrolysis decreases the molecular weight and crystallinity of cellulose and hemicellulose and thus leads to cellulose structure breakdown. But, even if the cellulose porous framework remains after chemical treatment, the framework robustness is an issue during carbonization at elevated temperature.
What is needed in the art is a method to selectively remove the lignin and leave the cellulose/hemicellulose framework behind, thus providing a MC structure constructed by carbonized cellulose framework without the drawbacks of current methods.